The present invention relates to a method of preparing a purine nucleoside compound at a high yield by utilizing an exchange reaction of nucleic acid base, which is carried out in the presence of an enzyme, and to microorganisms producing enzymes used for the preparation of the purine nucleoside compound.
Known is a method of preparing a nucleoside compound by utilizing an exchange reaction of nucleic acid base carried out in the presence of an enzyme. In the known method, however, equilibrium takes place between the starting materials and the reaction products, resulting in failure to improve the yield. For overcoming the difficulty, Japanese Patent Disclosure (Kokai) No. 4-197193 disclosed an exchange reaction of a base between inosine or 2'-deoxyinosine (hereinafter referred to as "deoxyinosine") and pyrimidine base, which is carried out in an aqueous solution of a phosphate in the presence of purine nucleoside phosphorylase (EC2.4.2.1) and pyrimidine nucleoside phosphorylase (EC2.4.2.2), and then hypoxanthine formed by the exchange reaction is converted into uric acid by xanthine oxidase so as to improve the reaction yield. In this method, since uric acid cannot act as a substrate of nucleoside phosphorylase, the reaction proceeds unidirectionally to form pyrimidine nucleoside.
However, JP 4-197193 noted above, which is directed to a method of efficiently preparing pyrimidine nucleoside by using inosine or deoxyinosine and pyrimidine base as starting materials, does not disclose a method of preparing a purine nucleoside compound by using a pyrimidine nucleoside compound and a purine base as starting materials which are readily available at low cost. Where a pyrimidine nucleoside compound is used as a starting material, it is necessary to add an enzyme serving to decompose the pyrimidine base formed by the base exchange reaction to the reaction system in order to deviate the equilibrium and to improve the reaction yield. However, preparation or isolation of the enzyme from natural product in a high activity state has not yet been reported. In other words, a method of preparing at a high yield a purine nucleoside compound using pyrimidine nucleoside compound as a starting material has not yet been established. Naturally, it is of high importance to develop a method of preparing a purine nucleoside compound stably at a high yield.